A Structural Model for Apolipoprotein C-II Amyloid Fibrils: Experimental Characterization and Molecular Dynamics Simulations

• Published in: Journal of molecular biology 2011-02, Vol.405 (5), p.1246-1266
• Main Authors: Teoh, Chai Lean, Pham, Chi L.L., Todorova, Nevena, Hung, Andrew, Lincoln, Craig N., Lees, Emma, Lam, Yuen Han, Binger, Katrina J., Thomson, Neil H., Radford, Sheena E., Smith, Trevor A., Müller, Shirley A., Engel, Andreas, Griffin, Michael D.W., Yarovsky, Irene, Gooley, Paul R., Howlett, Geoffrey J.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Amyloid - chemistry; Amyloid - ultrastructure; Apolipoprotein C-II - chemistry; atomic force microscopy; cross-β-structure; fluorescence resonance energy transfer; Humans; Microscopy, Atomic Force; Models, Chemical; Molecular Dynamics Simulation; Molecular Sequence Data; Protein Structure, Secondary; scanning transmission electron microscopy; Sequence Analysis, Protein; X-Ray Diffraction
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2010.12.006

The self-assembly of specific proteins to form insoluble amyloid fibrils is a characteristic feature of a number of age-related and debilitating diseases. Lipid-free human apolipoprotein C-II (apoC-II) forms characteristic amyloid fibrils and is one of several apolipoproteins that accumulate in amyloid deposits located within atherosclerotic plaques. X-ray diffraction analysis of aligned apoC-II fibrils indicated a simple cross-β-structure composed of two parallel β-sheets. Examination of apoC-II fibrils using transmission electron microscopy, scanning transmission electron microscopy, and atomic force microscopy indicated that the fibrils are flat ribbons composed of one apoC-II molecule per 4.7-Å rise of the cross-β-structure. Cross-linking results using single-cysteine substitution mutants are consistent with a parallel in-register structural model for apoC-II fibrils. Fluorescence resonance energy transfer analysis of apoC-II fibrils labeled with specific fluorophores provided distance constraints for selected donor–acceptor pairs located within the fibrils. These findings were used to develop a simple ‘letter-G-like’ β-strand–loop–β-strand model for apoC-II fibrils. Fully solvated all-atom molecular dynamics (MD) simulations showed that the model contained a stable cross-β-core with a flexible connecting loop devoid of persistent secondary structure. The time course of the MD simulations revealed that charge clusters in the fibril rearrange to minimize the effects of same-charge interactions inherent in parallel in-register models. Our structural model for apoC-II fibrils suggests that apoC-II monomers fold and self-assemble to form a stable cross-β-scaffold containing relatively unstructured connecting loops. [Display omitted] ► ApoC-II forms amyloid fibrils with one apoC-II for every rise of the cross-β-structure. ► There are two in-register parallel β-sheets stabilized by an internal ion pair. ► Each apoC-II molecule is stacked in a ‘letter-G-like’ configuration. ► MD simulations reveal a stable core and a flexible connecting loop.